Radiosurgery has typically been performed using a step-and-shoot approach that delivers radiation according to a three dimensional plan of multiple three dimensional shots. Multiple shots are usually required to destroy the target pathology. Step and shoot dose delivery involves repositioning the patient outside of the irradiation field to reposition the centroid of the conformal radiation dose. This type of radiosurgery has been time consuming and may in some cases have produced sub-optimal results.
Radiosurgery may be performed using various devices. For example, Leksell (Elektra, Stockholm, Sweden) provides a Gamma Knife™ which may be referred to as an LGK. The LGK provides accurate stereotactic radio surgical brain lesion treatment. The LGK derives its therapeutic radiation from 201 60Co radiation sources. A patient is exposed to these sources through pluggable collimator channels. The radiation beams passing through unplugged collimator channels focus in the center of a collimator helmet to create an elliptically shaped conformal dose distribution. LGK shots are traditionally elliptical due to the general shape of the human skull. For a single shot, dose drop-off is steep at the boundaries of this ellipse (e.g., 90% to 20% isodose). However, dose drop-off steepness is diminished and made difficult to estimate when two or more shots have overlapping dose distributions.
Shot planning seeks to achieve desired lesion coverage and killing. However, shot packing is not as simple as filling a tumor, a theoretical bag, with ellipses of dose. Planning multiple shots is difficult due to the consequences of unintended intersections of beams from different shots. These unintended intersections of beams from different shots complicate treatment planning, and thus lengthen the time required to plan a multi-shot treatment. Furthermore, shot packing approaches typically cannot commence until the entire pre-planning images are acquired.
Planning and delivery complexity are related to the geometric complexity and volumetric complexity of a target volume. For example, large lesion volume, complex lesion shape, and/or complicated geometric relationships between the lesion and critical structures complicate planning, and thus increase planning time and increase the likelihood that suboptimal results will occur.
Conventional LGK treatment planning begins with a treatment planning team that includes, for example, a neurosurgeon, a radiation oncologist, and a radiation physicist. The treatment planning team may survey pre-radio surgical images (e.g., CT, MR) to locate the lesion in a series of adjacent 2D image slices. Drawing the boundary of the lesion is referred to as “segmentation”. Other objects of interest, (e.g., critical structures near the lesion), may also be segmented at this time. Segmentation is typically performed manually using a contour drawing tool. Shot packing strategies may not begin until the entire set of image slices is available.
Conventional treatment planning falls into two categories: forward treatment planning, and inverse treatment planning, with forward treatment planning being the standard of care as of 2007. Treatment planning begins with known parameters including prescribed dose, lesion location, segmented tissue object contours, and so on. Forward planning includes a trial-and-error approach for choosing shot parameters including number of shots, shot positions, collimator sizes, shot weights, and so on. As shot parameters are selected the treatment planning team can calculate and evaluate the sum of the radiation dose distribution. The treatment team will then manually adjust setup parameters until an “acceptable” treatment plan is obtained. This is an extremely technical and manual process requiring the input of several highly skilled personnel. This approach is not deterministic.
Given time limitations imposed by single session treatment a significant issue for forward treatment planning is the relative size of the search and solution space for acceptable treatment plans. For a small lesion with a simple shape, forward planning may perform adequately. The treatment planning team may place a shot in the center of the target volume and then gradually add extra shots to fill the under-dosed regions closer to the lesion surface. However, the treatment plan search space increases dramatically when a lesion has a large target volume, a complex target shape, and/or a complex geometric relationship between the target volume and nearby critical section (CS). In this situation, treatment planning may require hours to obtain an acceptable treatment plan. The shots resulting from this trial-and-error procedure may produce unintended radiation dose overlap, particularly when multiple shots are placed in close proximity.